The present application is a U.S. national phase application of International Application PCT/EP99/05079 (International Publication WO00103900), filed Jul. 16, 1999.
The present invention generally relates to systems for measuring vehicle stability, and more particularly relates to a method and a related device for detecting variations of the center of gravity of the mass of a vehicle.
Various models for rollover accidents are described on pages 309 to 333, chapter 9, in xe2x80x9cFundamentals of Vehicle Dynamicsxe2x80x9d, by T. D. Gillespie, Society of Automotive Engineers, Inc., Warrendale 1992. Starting with a quasi-stationary model for a rigid vehicle, via a quasi-stationary model for a sprung vehicle up to dynamic models in consideration of inherent roll frequencies, conditions for existing rollover hazards are indicated.
While it was known at the time the above-mentioned book was published that in trucks, truck trailer units, buses, small buses, and off-road vehicles having a high center of gravity and/or a small tread width, there is a rollover risk during cornering in the event of a major roll motion, it has been found only recently that lateral motions of passenger vehicles are also likely to build up until the vehicle rolls over. Such a risk of rollover is considerably increased by improper loading, e.g. extremely toward one side or on the vehicle roof, because the center of gravity of the mass of the vehicle is shifted upwards or to one side.
As is shown in FIGS. 1a and 1b, the analysis of the forces and moments is decisive in the static case for the stable equilibrium of a vehicle.
FIG. 1a shows the distribution of the vehicle weight force FG to the wheel tread forces FAxe2x80x94xx. The lateral offset ys of the center of gravity S from the vehicle center is shown in FIG. 1b. The offset of moments with respect to a wheel shows the influence of the shift of the center of gravity in the static and dynamic case.
If, during driving operations, a disturbing force occurs, such as the centrifugal force during cornering or a wheel load variation due to a rough road section, the vehicle will move into the labile or unstable condition depending on the magnitude of the disturbing force.
With an Electronic Stability Program (ESP), for example, the system developed and sold by the applicant, a system is on hand which is apt to detect critical driving conditions by comparing the course of driving predefined by a driver with the actual track of the vehicle, and to prevent these conditions by a well-defined brake intervention. Wheel rotational speeds, the transverse acceleration, the steering angle, and the yaw rate are provided as data for this purpose. In case the driving condition defined by these data reaches a critical range, the proper steering behavior of the vehicle defined by the vehicle model of the ESP controller will be maintained by a wheel-selective brake intervention. Thus, this control action can only influence all two-dimensional processes and condition variables, that means processes which can be described by transverse acceleration and rotation about the vertical vehicle axis (yaw).
Once the position of the center of gravity in a longitudinal and/or transverse direction of the vehicle changes, e.g., due to loading, the changed cornering limit speeds are automatically taken into account in the ESP system because the proper steering behavior determined in the vehicle model shall be maintained. When the center of gravity shifts, e.g. in a rearward direction, the augmented oversteering tendency during cornering is counteracted by reducing the allowed cornering speed, in the event that the basic setting is established as understeering.
A change in the height of the center of gravity will now cause a changed side-tilt behavior of the vehicle in longitudinal and transverse directions. When a transverse acceleration acts on the vehicle, according to FIG. 1b, the analysis of moments
Fcxc3x97hs+Fnxe2x80x94rxc3x97b=FGxc3x97(b/2+ys)xe2x80x83xe2x80x83(1)
applies, wherein Fc designates the centrifugal force due to the transverse acceleration atransv., FG is the center of gravity, m is the vehicle mass, Fnxe2x80x94r,l designates the vertical wheel force (wheel load) for the right and the left wheel, hs, ys the height of the center of gravity, ys the lateral distance of the center of gravity from the vehicle center, and b designates the tread width. From this follows directly the correlation for the transverse acceleration atransv.
atransv.=1/hsxc3x97[g(b/2+ys)xe2x88x92Fnxe2x80x94rxc3x97b/m]xe2x80x83xe2x80x83(2)
Rollover of the vehicle will thus be reached when a critical transverse acceleration is exceeded for which the following correlation applies:
atransv.,critical=(b/2+ys)/hsxc3x97gxe2x80x83xe2x80x83(3).
Thus, the critical transverse acceleration depends directly on the center of gravity.
Besides, in prior art vehicle motion control systems, wherein the objective is to increase driving stability, especially during cornering, that means to avoid lateral rollover in particular, it is generally the transverse vehicle acceleration or the roll angle of the vehicle which is taken as a basis for critical condition variables that describe the rollover hazard.
Thus, DE-A 197 46 889 describes a system to increase the lateral stability during cornering which is equipped with a side-tilt detection device. This side-tilt detection device measures either the difference in height between the right and the left vehicle side, or the transverse acceleration of the vehicle in order to detect the roll angle between the vehicle horizontal and the roadway horizontal. When the side-tilt detection device detects a rollover hazard, a countersteering yaw torque will be produced by decelerating the curve-outward front wheel.
However, as described hereinabove, the allowed transverse acceleration and the allowed roll angle are greatly responsive to the position of the center of gravity of the vehicle, especially the height of the center of gravity. In the generic methods and devices known from the state of the art, the center of gravity of the vehicle, especially variations in the center of gravity, is not taken into consideration to a sufficient degree as far as a preventive rollover prediction is concerned.
Please delete the abstract on page 13 and replace it with the abstract set forth immediately below in clean form. Additionally, in accordance with 37 CFR 1.121(b)(iii), all paragraphs amended herein are set forth in a marked up version on the sheets attached to this amendment.
Therefore, an object of the present invention is to provide a method and a device which render it possible to counteract a rollover hazard as early as possible. The primary objective of the present invention is a prediction of an imminent rollover condition of a vehicle to permit a best possible prevention.
According to the present invention, this object is achieved in that first condition variables which correspond to the respective wheel load are detected on at least two wheels during cornering, in that the detected first condition variables are compared with reference values representative of the respective cornering maneuver, and in that a related variation of the center of gravity is calculated from the differences between the detected first condition variables and the reference values.
The present invention is based on the recognition that the most appropriate coefficient of influence in a best possible preventive detection of the rollover hazard of a vehicle is the position of the center of gravity of the vehicle. Thus, the current center of gravity as an output quantity is made the important basis of the method and the device disclosed in the present invention.
The coefficient of influence directly indicates the existence of a critical rollover condition. Either active control interventions, e.g. by means of ESP, or a passive warning to the driver will be considered as a countermeasure to prevent rollover.
In contrast to the state of the art, the preventive procedure disclosed by the present invention includes the advantage that the vehicle remains steerable during a correcting intervention required and the driving comfort is also preserved during an active control intervention. Another advantage of the present invention over the state of the art can be seen in that the possible intervention strategies in response to detection of a critical driving situation are almost optional, not least due to the detection which offers extreme prediction. Therefore, the present invention can be realized without special effort and cost as an extension or an improvement of an existing driving stability control system, for example, the Electronic Stability Program (ESP) of the applicant.
The spring travel that can be measured at the wheel suspension, the spring pressure, the damper pressure that can be measured at the shock absorber, the inside tire pressure, or the lateral deformation of the tire can be made the basis of a first condition variable that is correlated to the wheel load.